Re: 2013 Australian Open Wildcards.
Plenty of different reasons for quitting, but the main reason is financial, then the realisation that you can only reach a certain level in the tennis world, and that is not going to keep you fed and build for the future, so you have to start looking at other opportunities and/or options.
Playing on the circuit is a huge commitment, in time, effort and money and you can't perform at the highest level if that motivation drops...
Injuries, even if niggling, play their part and affect your fitness, Lose fitness and then it is another up hill battle to just get back to where you were, never mind improve.
Motivation, desire, support system all play their part.
Good luck to James if he quits and even better luck to him if he continues..